The use of a Penning gauge as excitation source for the measurement by optical spectroscopy of the partial pressures of tokamak exhaust gases is well known. At JET, the technique is used to study the efficacy of various divertor configurations in removing He from the core plasma. The light intensity from the Penning discharge is measured at two transition wavelengths, those of Dalpha and HeI at 656.1 and 587.6nm, respectively. In the range of 10-6 to 5x10-3 mbar, approximately, the intensity in each channel is proportional to partial pressure. A complication is that the He signal is polluted by molecular emission from the D2. During system calibration, without plasma, using pure D2 the intensity in both channels is recorded versus pressure, giving the response to this gas and the cross talk in the He channel. The process is repeated with a known concentration of He (typically ~10%), to yield the system response to that gas. From these data, by taking ratios of intensities versus pressure, coefficients are derived with which to analyse signals obtained with plasma.